RESULTS

The following measurement examples were carried out with one CR injectors. Figure: 4 shows a typical force time history of a global spray momentum measurement at different distance from the nozzle. As expected, increasing the nozzle distance, the force signal is delayed and less intense.

RESULTS

0 1 2 3 4 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Time, ms Target distance from nozzle, mm

Figure: 4 — Example of global spray momentum measurement

By the time integration of the force curve we can calculate the spray momentum and we can plot this value at difference distances as shown in the following plot.

Increasing the counter pressure the spray momentum decreases more quickly due to the intensification of the atomization process.

This approach in the evaluation of the global momentum flux in steady state conditions can be used to derive interesting awareness about flow emerging from

The nozzle. If additional hypotheses are assumed — uniform flow velocity veff at the nozzle exit section Aeff, separation between the regions occupied by the two- phases and negligible contribution of the vapor phase to the spay momentum, the instantaneous mass flow rate m and momentum flux M can be expressed as :

TOC o "1-5" h z m = plveffAeff M = Plv^ffAeff (2)

Hence, the effective flow discharge velocity veff and the effective flow section Aeff can be evaluated by;

M. m2

Veff = m ‘• Aeff-ppM (3)

From the Eq. (2) and (3), defining the global discharge coefficient as Cd — Cv ■ Cs the following definitions for the velocity reduction Cv and flow section reduction Cs coefficients follow:

C = veff = veff. C = Seff (4)

Cv = jmp" v„’ Cs — So <4)

The correct application of the previously described analysis requires that steady state flow conditions are achieved, and to this end (for the case of common-rail injection system) energizing time settings typically ranging from 3 ms to 5 ms are applied, which are quite far from engine-like operating conditions. Further, for the momentum flux evaluation, only the mean value of the spray impact force during the middle part of the injection process (i. e. skipping both the injection on-set and closing transients) is considered. This testing procedure is to be considered fully appropriate if the subject of the analysis is actually the nozzle behaviour in terms of discharge capabilities and cavitation intensity in steady needle lift conditions. On the other hand, the steady flow momentum analysis cannot give any significant information about the injection system behaviour during injection events that do not achieve steady conditions, which is the most typical operation mode for automotive Diesel injection systems. For a transient injection analysis the complete momentum conservation equation (Eq. (1)) must be applied to the fluid control volume: the inertia of the fluid inside the control volume and the possibly non-zero velocity components of the fluid exit the control volume itself could be significant terms that affect the coincidence between what is measured in terms of impact force time-history and the momentum flux time-history at the nozzle exit.

In previous papers, the Authors discussed the application of the impact force measurement as an indirect method to estimate the spray global momentum flux also in transient conditions [12, 13] and for local measurement.

The local spray momentum measurement is the measurement of a small spray "tube flux" in order to characterize the force impact profile inside the single spray. The measurement of momentum flux of a small spray portion is obtained by means of a particular device, called "local device", described before, see Figure: 3.

This device is composed by a conical screen with an hole on the top; inside the hole is inserted a pin of 1 mm diameter which is supported on the sensor. Therefore the system measures the force impact of a small spray portion whereas all the spray is deflected by the cone screen. Moving this device inside the spray the instrument can measure the spray momentum distribution at one distance from the nozzle. The analysis can be repeated at different distances from the nozzle and for each hole.

RESULTS

Figure: 5 — Example of spray momentum distribution of CR Diesel injector

The instrument is able to carry out this measurement with an high spatial resolution and relatively quickly; A complete injector scan is composed of about 5000 measurement points and takes about 6 hours. By means of the analysis of this distribution it is possible to get many information; in fact the spatial integration of the curve is it done in order to calculate the volume underlying at each distance and the value of the integration is reported for each hole of the nozzle in a graph.

Hole to hole comparison in terms of spray momentum at different distances is shown in that graph. The same information can be reported with the bar chart of Figure: 6 in which is reported the difference for each single hole of the nozzle versus the "average value of all holes" for each distance. By this graph is it possible to see that the hole n°1-7-8 are richer in momentum than the average and the holes n° 4-5 are poorer. This trend is confirmed by images shown in the same figure. By this approach the instrument is able to emphasize the non-uniformity of the holes in terms of spray momentum. Moreover analyzing the spray momentum distribution the spray axis position at different distances are recognized; in this way the real spray axis trajectory is recognized and it is compared with the theoretical direction; this kind of analysis is showed in Figure: 7.

As reported in the graph it is possible to have a difference between theoretical and real axis of some degree that can be fundamental in the injection system developing.

RESULTS

15 20 25

Z Distance from nozzle, mm

□ 10 mm 15 mm 20 mm

— 1-

II

Il

N

1

2

Is!

4

L5|

U6

7

8

LI

Hole Number

15 10 # 5 e c en 0 iffer S -5

-10

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-10
-15
Image delay: 420 (js from ET Image delay: 520 |js from ET

Jet 1

Jet 2

Jet 8

„„ — Vi/*. «3

Jet 4

подпись: jet 4‘♦ /IX

Jet 6

Jet 5

Figure: 6 — Spray momentum spatial integral vs distance (up); hole to hole spread (down). Spray global image at two different delays

■ Phi Teo

Radial Direction, mm

подпись: 
radial direction, mm
Hole 1

•Hole 2 » Hole 3

•Hole 4

•Hole 5 » Hole 6

•Hole 7

•Hole 8

30

V

11

V/

Tl

10

V/

T

0

, j.1 ^

30 -2

0 10′

) i

0 2

03

10

20

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-30

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•-f-

X Direction, mm

Hole 1 Teo Hole 2 Teo Hole 3 Teo Hole 4 Teo Hole 5 Teo Hole 6 Teo Hole 7 Teo Hole 8 Teo Hole 1 Hole 2 Hole 3 Hole 4 Hole 5 Hole 6 Hole 7 Hole 8

подпись: hole 1 teo hole 2 teo hole 3 teo hole 4 teo hole 5 teo hole 6 teo hole 7 teo hole 8 teo hole 1 hole 2 hole 3 hole 4 hole 5 hole 6 hole 7 hole 8

E

E

E

Io

Ti

C

E

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подпись: e
e
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Figure: 7 — Theoretical and real axis comparison

The measuring of real axis direction and the hole to hole comparison are the most important output of the test system because they are strongly useful in order to optimize the drawing of combustion chamber and for the developing of injection system. Moreover this kind of analysis can be used as quality test of the injector production in order to increase the quality standard of this component.

Another kind of analysis can be done by the study of the spray momentum distribution in a plane parallel to the axis of the injector in order to visualize the evolution of force impact along the nozzle distance. Based on this representation it is possible to operate a section on this graph in order to have a profile of the momentum of the spray along a radius of a spherical cap.

RESULTS

Figure: 8 — Example of an axial spray momentum section (left) and of a spray momentum profile (right)

Considering this profile it is possible to define a threshold and identify the intersection with the profile; the threshold can be selected considering different criteria. Normally a percentage of the maximum profile value at one distance (15 mm) it is considered as threshold. In this way it is possible to calculate the spray angle based on the spray momentum measurement but it is necessary to consider that this angle is usually lower than the typical cone angle values calculated with visualization devices because the external spray portion reflects light but they are very poor in spray momentum quantity.

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